Presently, there are two principle techniques used for commercial fabrication of optical fiber preforms--the "soot deposition" process and the modified chemical vapor deposition (MCVD) process. These processes involve the thermochemical production of glass from appropriate glass precursor vapor.
In the soot deposition process, the glass precursor vapor is introduced into a hydrolyzing flame and particulate material, commonly referred to as soot, is formed. The stream of particulate material emanating from the flame is directed toward a mandril, which may be, for example, a cylindrical tube or a glass rod, on which the soot is deposited. Following such deposition, the soot is consolidated into a transparent glass, the mandril removed and the resultant hollow tube collapsed to form a solid, cylindrical optical fiber preform.
In the MCVD process, which is summarized in an article titled "Reproducibility of Optical Fibers Prepared by a Modified Chemical Vapor Deposition Process" by F. V. DiMarcello et al. in the Bell System Technical Journal, Vol. 57, No. 6, July--August 1978, page 1723 et seq. a stream of glass precursor vapor is directed through the center of a glass tube. The tube is usually composed of glass material which may be appropriate for use as a cladding in the fiber. The tube is heated, causing the gas vapor to react with the inner wall of the tube to cause a glass layer to deposit thereon. The absence of a flame in direct contact with the gas results in a preform having high glass purity and low optical losses. When a sufficient number of such glass layers have been deposited on the inner wall of the tube, the resulting cylinder will also be collapsed to form the solid preform having a substantially cylindrical core encased within a surrounding cladding.
In both methods, a number of sequential passes are made to deposit a plurality of concentric layers of material to obtain the desired thickness. All layers may have the same refractive index resulting in a step index preform, or each layer may have a slightly different refractive index resulting in a graded index preform. The graded index preform will have an increasing refractive index towards the center of the preform resulting in a substantially parabolic refractive index profile.
The refractive index profile of an optical fiber preform is expressed, by those knowledgeable in the art, in the form of ##EQU1## where: n=refractive index
r=radius position in preform PA1 a=radius of the core PA1 n.sub.o =at r=a, at the edge of the core region, typically the cladding refractive index PA1 .gamma.=refractive index gradient power PA1 .DELTA.=refractive index ratio.
Those skilled in the art use the refractive index gradient power .gamma. and the refractive index ratio .DELTA. to determine the character and acceptability of the profile, n(r), of the graded index preform. Variations in the parameter .gamma. result in a change in the shape of the desired parabolic profile while changes in the parameter .DELTA. will cause proportionate changes in the height of the profile.
The refractive index profile of an optical fiber is substantially the same as the profile of the preform from which it was drawn. Therefore, deviations from the parabolic refractive index profile n(r) of the preform may result in unacceptable transmission characteristics of the fiber drawn therefrom. Thus, it becomes most advantageous to determine the parameters .gamma. and .DELTA. to characterize the refractive index profile of the preform prior to drawing fiber therefrom to avoid the expense of producing kilometers of optical fiber that would be unacceptable for its intended use.
One method for determining .gamma. and .DELTA. of a drawn fiber is described in an article titled "Automatic Analysis of Interferograms: Optical Waveguide Refractive Index Profiles" by B. C. Wonsiewicz et al. in Applied Optics, Vol. 15, No. 4, April 1976, page 1048 et seq. That article describes a method which provides a computer-generated plot of the refractive index profile n(r) versus the radius of an optical fiber. The data are obtained from the pattern of a thin, polished cross section of a fiber as viewed in an interference microscope. The interference pattern is digitized with a scanning microdensitometer, followed by a computer determination of the position of the center line of each interference fringe. A computer program determines the value of .gamma. corresponding to the curve that best fits the data in the aforementioned equation for the profile n(r). The value for .DELTA. is determined from the maximum of the profile n(r) of the plot. Although such a method can effectively determine the profile of the optical fiber, it is destructive, time consuming and expensive.
Nondestructive methods using diffraction techniques such as described in an article titled "Refractive-index profile determination of optical fibers, from the diffraction pattern" by Ernst Brinkmeyer, in Applied Optics, Vol. 16, No. 11, November 1977, pages 2802 and 2803, have been used to obtain the refractive index profile of an optical fiber. However, as previously indicated, determining the parameters after the fiber is drawn can be wasteful of time and material.
Thus, there is a need for a non-destructive, non-contact method of determining the refractive index profile parameters of an optical fiber preform.